Spontaneous daily torpor is an endogeneous daily rhythm which is associated with winter survival of the white-footed mouse, Peromyscus leucopus. Recent studies confirm that seasonal change in photoperiod (12 hours of light/day or less) is a principal exogenous influencing occurrence of torpor and that the pineal gland is involved in mediation of this effect. Pinealectomy blocks incidence of daily torpor while replacement therapy with melatonin restores torpor. Thus, in the study of daily torpor there exists a possibility for linking melatonin action and seasonal photoperiodism with an endogenous daily event only expressed under a short day photoperiod. This study proposes: 1) determining the daily pineal melatonin rhythm in mice exposed to a long and short day photoperiod (using radioimmunoassay), 2) examination of the role of this rhythm on daily torpor by experimental manipulation of the melatonin rhythm (using programmable miniature osmotic pumps), 3) an initial investigation on the possible sites of melatonin action in the brain with emphasis on thermoregulatory centers (using autoradiography and hormone binding studies), and 4) the use of intraspecific differences for photoperiodism (present in Connecticut, but absent in Georgia mice) to confirm the role of melatonin in this species.